1. Field of the Invention
This invention relates to a process for preparing 1,4-dienes of high trans-isomer content.
2. Description of the Prior Art
Sulfur-curable elastomeric copolymers of .alpha.-olefins with nonconjugated dienes are well known. Particularly important are terpolymers of ethylene, propylene and a nonconjugated diene comonomer having only one polymerizable double bond. Such terpolymers are known in the industry as EPDM elastomers. The nonconjugated diene comonomer generally is a 1,4-diene such as 1,4-hexadiene. Terpolymers of this type are finding increased use, for instance, in manufacture of molded automobile parts, transmission belts, hoses, and the like.
1,4-Dienes can be prepared by several processes, including the catalytic addition of an .alpha.-olefin to a conjugated diene. U.S. Pat. No. 3,306,948, to Kealy discloses such a catalytic process. In this process the reactants are contacted in the presence of a catalyst made from at least two moles of an organometallic compound and one mole of a nickel compound containing at least one monodentate trivalent phosphorus ligand such as tributylphosphine. The organometallic compound can be, for example, an aluminum alkyl, an aluminum aryl, or an organoaluminum halide.
Although these processes give high yields of 1,4-dienes, they are deficient in that 1,4-diene products of low trans/cis isomer ratio, generally about 2:1 to at most 3:1, are obtained. Trans-1,4-dienes are much more desirable monomers for the EPDM elastomer synthesis because they give straight chain copolymers of good physical properties and sufficient unsaturation for sulfur vulcanization. The cis isomers give less unsaturated copolymers, thus rendering the copolymers less attractive commercially.
U.S. Pat. No. 3,565,967, to Collette and Su discloses a process for obtaining 1,4-dienes having a trans/cis-isomer ratio of at least 4:1. In this process ethylene is contacted with a 1,3-diene in the presence of a soluble zerovalent or divalent nickel compound, an organoaluminum chloride or bromide, and a tertiary phosphine ##STR1## WHERE A is the naphthyl radical, B is either a C.sub.1 -C.sub.6 alkyl or the allyl radical, D is either the phenyl or a substituted phenyl radical, m+ n+ p= 3, each of m and n independently can be 0, m cannot be larger than 1, while p cannot be smaller than 1, provided that when D is pentafluorophenyl, pentachlorophenyl, or tetramethylphenyl, then p must be 1. Further increase in the trans/cis ratio can be obtained by the addition of an aluminum compound of the structure (R.sub.1).sub.a AlZ.sub.b where R.sub.1 is an alkyl, cycloalkyl or aryl radical having 1-12 carbons, and Z is --OR.sub.2, --N(R.sub.3)(R.sub.4) or =N(R.sub.5) where each of R.sub.2, R.sub.3, R.sub.4, and R.sub.5 can be an alkyl, cycloalkyl, aralkyl, or aryl radical having 1-12 carbons; when Z is =N(R.sub.5), R.sub.1 can also be hydrogen; each of a and b is either 1 or 2, and the sum of a+ b is 3; except that when Z is =NR.sub.5, each of a and b is 1, and a+ b is 2. The preferred phosphine, (C.sub.6 H.sub.5).sub.2 PC.sub.6 F.sub.5, is relatively expensive and its use generally leads to slower reaction rates. It would be desirable to find a catalyst component which gives more rapid reaction and is less expensive than the tertiary phosphine.